Along with the continuous development of the industry of power sockets and plugs that have leakage protective functions, higher and higher requirements are proposed for the safe utilization of power sockets and power supplies with leakage protective functions. People hope that it should be possible to examine regularly if the leakage protective plug/socket has a leakage protective function while the plug/socket is in service. In this way, when its life terminates, i.e. the internal elements are rendered ineffective and the leakage protective function is lost, the user can be reminded in time to replace it with a new product.